1. Field of the Invention
The present invention relates generally to the field of electrical current sensing devices such as those used in programmable logic controllers and other environments. More particularly, the invention relates to a technique for sensing current with very high resolution in a device which can be at least partially formed in large quantities through micro-machining and similar techniques.
2. Description of the Related Art
A wide range of applications exist for devices capable of accurately sensing electrical current. In certain applications, current is sensed for simple readout, such as on a metered scale or digital display. A considerable number of applications, however, require current to be sensed for use in regulation of power or as feedback for the control of machines, circuits, and processes.
In the industrial environment, for example, feedback devices and actuators typically operate within specified current and voltage ranges. Feedback from sensors may assume values within the acceptable range, with the values reflecting physical parameters of a controlled system. One such application is in programmable logic controllers (PLC""s) in which a 4-20 ma current range is typically provided for control and feedback. To enhance the performance of systems incorporating these devices, it is often desirable to obtain very high resolution current sensing in a manner which avoids unnecessary power drain from the associated circuitry or system.
Digital current sensors presently in use in applications such as PLC""s suffer from several drawbacks. In certain known 4-20 ma current sensors, for example, isolation from perturbations which may be caused by external circuitry is achieved by electrically xe2x80x9cfloatingxe2x80x9d an analog-to-digital converter and its associated electronics. This involves a floating power supply circuit which uses a DC-to-DC converter which receives an input from a 5 volt DC power supply, converts it to an AC signal, transforms the AC signal for isolation, and reconverts the AC signal into DC power. The isolated electronics then communicate with non-isolated electronics through a pair of digital opto-isolators. These isolators are then coupled to a clock and to an output which returns digital values in serial form. However, opto-isolators used in such devices are, in general, unsuitable for use in an analog fashion for resolutions higher than 6-8 bits, due to their temperature dependency and drift. Moreover, to reduce costs in such sensor circuitry, analog inputs for a single module are generally not isolated from one another so that they can share a same floating power supply.
An important drawback in present state-of-the-art current sensors is their unit cost. Even in applications where a single current sensor suffices for feedback or control, sensors of the type described above can add significantly to the overall system price. Moreover, in many PLC applications, it is desirable to provide current sensors on many or all outputs of a PLC to monitor output current for system control, as well as for diagnostic monitoring, such as for output protection. As noted above, similar high resolution, low cost requirements exist for sensing on input channels of PLC""s and other devices.
Still further drawbacks in existing technologies include inconsistencies in device-to-device performance, in compatibility with microelectronics, parasitic losses, and the physical dimensions of the current sensor and associated circuitry. These and the foregoing drawbacks can further lead to problems with energy dissipation, heating and other thermodynamic and performance problems.
There is a need, therefore, for an improved technique for providing high resolution current sensing at a relatively low cost for both the sensing device and its associated circuit. There is, at present, a particular need for a current sensing technique which offers enhanced performance in a reduced package size, facilitating both manufacturing and incorporation into electrical and microelectronic devices such as PLC""s.
The present invention provides a current sensing technique designed to respond to these needs. The technique is particularly well suited to floating current sensing, such as in a floating current input stage or an output stage of a PLC. However, the technique may be used in a wide range of devices, both for control, feedback and monitoring functions. The technique employs microelectro-mechanical systems (MEMS) features to form a small, cost effective unit which may be employed in single modules or in module arrays.
The basic module formed in accordance with aspects of the present technique includes a deflectable member disposed in a magnetic field. A current to be measure or sensed is applied to the deflectable member, urging the deflection of the member by interaction of the magnetic field with the electromagnetic field produced by the flowing current. Feedback or output devices are coupled to the deflectable member. In a preferred embodiment, the feedback device includes a capacitor, output voltage of which varies with deflection of the member.
The deflectable member and the feedback, or output device may be employed either in an open loop or a closed loop setting. In a particularly preferred configuration, a pair of additional deflectable members flank the deflectable member to which the sensed current is applied. These additional members are mechanically linked to the sensed current member. Feedback devices, such as micromachined capacitors may be provided on either side of the additional deflectable members. Nulling currents may be passed through the additional deflectable members to counter the deflection of the sensed current member. By driving the feedback to a null level or into a known tolerance range, the current applied to the sensed current member is determined based upon the level of the nulling current. In an open loop configuration, the output of one or more feedback or readout devices, such as micromachined capacitors, may be evaluated directly to provide an indication of the sensed current.
The technique provides significantly enhanced resolution as compared to heretofore known devices. For example, a current sensor in accordance with the present technique may provide output resolution of 12-16 bits over an operating range of 4-20ma. It does not require a floating power supply, and analog inputs from single modules may be isolated from one another without additional expense. Where desired, arrays of large numbers of the modules may be easily formed and coupled to one another for enhanced performance.